A gas chromatograph (GC) is an instrument used to spatiotemporally separate and detect gas phase mixtures by passing sample plugs through a channel (i.e., the column) coated with a functional material (i.e., the stationary phase). The constituents can be identified by the time taken to elute from the column and quantified by the strength of the signal from a gas detector located downstream of the column. In general, many other components are also integral to the operation, such as the preconcentrator that provides sample injection and the pump that generates the gas flow. In some systems, valves are used to control the timing and direction of the flow. The separation of complex mixtures is sometimes performed using comprehensive two-dimensional GC (2DGC or GC×GC), in which a thermal modulator is used.
Since the widespread adoption of the gas chromatograph by the petroleum industry in the 1950s, its use has been extended to a number of other fields. For example, it is used to examine pollutants, such as polycyclic aromatic hydrocarbons, pesticides, halogenated compounds, etc. Another application is food analysis: coupled with the solid-phase microextraction technique, it is used for the identification and quantification of lipids, drugs, pesticides and carbohydrates. In recent years, biomedical screening has also been performed by this instrument. The analysis of human exhaled biomarkers by the GC provides a non-invasive approach for diagnosis and monitoring of potential diseases. Examples of such biomarkers include nitric oxide related to pulmonary inflammation, and ethane and pentane related to lipid peroxidation.
The miniaturization of the GC has been an ongoing effort for over 30 years, with early work dating back to 1979. As the core component of a micro gas chromatography (μGC) system, the separation column has been widely investigated and various column structures have been reported, such as the nickel column, the silicon-glass column, the Parylene™ column and the plasma-enhanced chemical vapor deposition (PECVD) oxynitride column. The stationary phase coating methods for these columns include the conventional static coating method as well as a self-assembly process. The gas injection device for a μGC system can be mainly classified into two categories: the preconcentrator and the valve injector. The preconcentrator uses sorbents to collect analytes at low temperatures and inject a plug with a thermal pulse. Certain preconcentrators collect analytes without the need for gas flow. Conversely, the valve injector utilizes valves to sample and inject a plug of gas. A variety of gas detectors have been reported, including the chemiresistor, the chemicapacitor, the thermal conductivity detector, the Fabry-Perot detector, and the discharge-based detector. A microfabricated thermal modulator has also been reported along with its application in a GC×GC system.
The integration of the microfabricated components into a μGC system has also made remarkable progress. The μChemLab is a hand-held μGC system that consists of a preconcentrator, a column, and surface acoustic wave sensors. Researchers at the University of Michigan have reported several prototypes of μGC over the past decade, including the Intrepid, the Spiron and the palm-size Mercury system.
Most μGC research efforts have not incorporated the use of micropumps. Only two cases have been reported: one with a microfabricated, electrostatically-actuated peristaltic pump and another with any array of motionless Knudsen pumps. The former required high frequency, large amplitude, drive voltages but was power-efficient. The latter was not power-efficient, but required only a low-voltage DC source; it provided high reliability, with continuous operation over 6000 hours.
Many micropump-operated μGC systems reported to date have used components fabricated by disparate microfabrication processes. Some systems connect the components by tubing, whereas some use manifolds for fluidic interconnect. The benefit of this approach is that each component can be optimally designed and fabricated. Unfortunately, the increased complexity and cost of the fabrication of the whole system pose a challenge for integration. As in other fluidic systems, a stackable architecture or a monolithic process for all of the components would greatly benefit the manufacturability and integration of the system.
This section provides background information related to the present disclosure which is not necessarily prior art.